Method and apparatus for controlling pulp refiners

ABSTRACT

A control system, receiving inputs from a group of grinders or refiners, generates a signal representative of overall grinding or refining action. Operating from this signal, the control system controls at least one of the grinders or refiners in the group such that the cumulative qualitative output from all the grinders or refiners is equal to a desired set point.

LATA -r D United States Patent 11113,568,939

[72] Inventors Donald B. Brewster [56] References Cited y York UNITED STATES PATENTS F' 2,699,095 1/1955 Irwin 241/37x Cheng S. Lin, Charleston, S.C.

RE24,185 7/1956 Staege... 241/37X [21] Appl. No. 775,975

2,887,277 5/1959 Sakata 241/37 [22] FM 1968 3 117 734 1/1964 M c n 241/29 45 Patented Mar. 9 1971 1 c a Y 3,309,031 3/1967 McMahon 241/37 [73] Ass1gnee Westvaco Corporation New York, N.Y. Primary Examiner-Donald G. Kelly Attorneys-Robert S. Grimshaw and Alfred L. Michaelson [54] METHOD AND APPARATUS FOR CONTROLLING PULP REFINERS l2 Chums 8 Drawing ABSTRACT: A control system, receiving inputs from a group [52] U.S. Cl .7 241/28, of g i 0r refiners, generates a gn l r presentative of 241/30, 241/37 overall grinding or refining action. Operating from this signal, [51] Int. Cl ..B02c 25/00, the control system controls at least one of the grinders or B02c 21/00 refiners in the group such that the cumulative qualitative out- [50] Field of Search 241/28, 30, put from all the grinders or refiners is equal to a desired set 33-37, 146 point.

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AGENT PATENTED MAR 9197! sum 5 hr 8 kUQkM METHOD AND APPATUS FOR CONTROLLING PULP REFWERS BACKGROUND OF THE INVENTION 1. Field to Which the Invention Pertains Numerous characterized operations utilize grinding or refining processes which are generally characterized by an apparatus which performs work upon a material supplied to the apparatus. in general, it is the object of these processes to alter some property of the material by performing work thereon. In connection with these processes, a control problem is presented in that the apparatus which performs the grinding or refining operation must be controlled to insure that the amount of work performed is such that the desired property level is obtained. A particularly difficult control problem is presented when the grinding or refining processes are operating in parallel.

Exemplary of such an operation is the refining of paper pulp in the paper industry wherein the pulp must be refined or worked upon prior'to being supplied to the paper machine. Since paper pulp refining is so illustrative of the control problems associated with the parallel operation of grinding and refining processes, we will, by way of example and not by way of limitation, consider the application of our invention to a paper pulp refining operation.

2. Prior Art Since our invention relates to the control of refining processes and particularly paper pulp refiners, rather than the refining apparatus itself, a detailed description of the objects of refining, the apparatus used and the methods employed will not be undertaken. Suffice it to say that, in general, a pulp slurry is fed to a refiner wherein it passes between two surfaces, a velocity differential existing between the two surfaces. The refiner is provided with means to adjust the distance between the two aforementioned surfaces which means may, generally speaking, be either hydraulic, pneumatic, mechanical or a combination thereof. Further, the refiner is also provided with means which create and maintain the velocity differential between the two surfaces, as, for example, an electric motor.

By way of example, the two aforementioned surfaces might be either two discs, one of which is axially movable with respect to the other or the so-called conical type wherein a plug is axially movable within a shell.

As the pulp slurry passes between the two surfaces, a certain amount of work is performed on the slurry. Later in the papermaking process, the amount of work performed upon the slurry is manifested by the ability of the formed paper web to retain the water which, along with pulp, had comprises the slurry. This property of the newly formed web which, as we have said, is a manifestation of the work performed upon the pulp by the refiner, is used by those skilled in the art as an index of refining action and is commonly referred to as freeness.

Since freeness is such an important property of the web, it becomes desirable to insure that the freeness of the stock supplied to the paper machine is closely controlled. In attempting to closely control this important variable, problems have arisen largely because of the elapsed time (which is in the nature of a mixing and transport delay) between the point where the freeness measurement is made of the paper machine and the refining operation. That is to say, if for some reason the amount of refining action changes, a significant amount of time may elapse before this change is fully sensed by the freeness measurement on the paper machine, e.g. the elapsed time may be as great as 30-45 min.

ln attempting to solve the problem of controlling freeness, which is, in reality, done by controlling the refining operation (other factors being constant), the prior art has contrived several approaches. The following brief summary of some prior art attempts to solve this control problem will illustrate those aspects of the problem which the prior art had not solved but which are solved by our invention, thereby providing a more clear and concise description of our invention.

Among the earlier attempts to control freeness were systems wherein a signal, representing freeness, was derived at the web forming portion of the paper machine. Either the signal thus obtained or an error signal resulting therefrom was transmitted directly to one or more refiners where it was used to control the amount of refiner action by adjusting or readjusting the distance between the refining surfaces. Although this approach provided a measure of control, its capabilities were usually limited. Since the control feedback loop spanned a portion of the process havingan extremely long time delay, i.e. the aforementioned mixing and transport delays, the maximum sensitivity of the control had to be rather low for the system to remain stable.

Recognizing the sensitivitystability impasse created by the above approach, resort was made to a cascade type of approach. Of course, to achieve this approach, local or secondary loop control of the refining operation was required, i.e. it was required that there be a secondary control loop or system onto which the primary control signal resulting from the freeness measurement could be cascaded.

Of course, to establish any such local'or secondary loop control, it was necessary to determine what, if any, variables existed within the refining operation which were both indicative of refining action and measurable. The prior art selected these variables upon the realization that refining action (which is manifested by or controls freeness) was related to the work done on the pulp as it passed through the refiner. From this realization the prior art was lead to the postulation that a measurement of the work input to the refiner system could be used as a local index of refining action. Thus, the power consumption of the electric motor moving the refining surfaces relative to each other was used to measure refining action. With this local signal available, individual control loops were utilized to control each refiner, all the local loops having a common set point which was derived from the freeness measurement on the paper machine.

As an alternate to using motor power consumption as an index of refining action, thermodynamic considerations lead the prior art to the realization that a measurement of the temperature rise of the pulp, across the refiner would be an equivalent manifestation or index of work done and therefore refining action.

This temperature rise, commonly referred to as refiner AT, was utilized in a manner similar to motor power consumption, i.e. individual control loops on each refiner were employed to control refining action, again, all of the individual loops operating from a common set point which was derivable from the freeness measurement.

In considering the refiner control systems disclosed by the prior art and described above, we perceived that all such prior art control systems have been predicated upon the premise that the refining action of all refiners must be equal. The following example will more fully describe this perception.

Assume that a group of refiners are operating to supply stock to a paper machine, it being desired that the stock have a freeness X. All prior art control systems have had as their objective the control of each, individual refiner in the group of refiners on an individual basis. Since each of the individual refiners in the group had its own control system or loop, the control strategy was to insure that the freeness of the stock from the individual refiners was equal to X. Since the freeness of the stock from each refiner is directly relates to the refining action of each refiner and since the refining action may be measured by, e.g. the temperature rise of the pulp across the refiner (AT), it will be appreciated that all prior art control systems were employed to insure that the AT across all of the individual refiners were equal.

While one can control a refining operation with the ap-. proach of using individual control loops on individual refiners to insure that the output of each individual refiner has a freeness equal to the desired overall freeness, such an approach imposes many limitations on the refining operation.

For example, it is the usual practice for each of the individual control loops to have its own set point. Thus, if it is required or desired to change the freeness, an operator would have to go from refiner to refiner changing set points. Such a procedure is not only a source of errors but because of the time required the transient period during which the change takes place is extended. Further, the more refiners on line, the greater will be this transient period.

Another limitation of the approach of individual control loops is in the manner or sequence in which the refiners are loaded. In a typical refiner arrangement (as will later be illustrated in FIG. 1 of our drawings), a number of refiners will be operating in parallel. The stock output of all the refiners will be fed to a mixing tank known as a machine chest. Interposed between the outlet of the machine chest and the paper machine headbox may be a so-called tickler refiner which will further refine at least some of the stock going to the paper machine. The use of a tickler refiner is significant in that it is located after the stock chest. Since the mixing delay which occurs in the machine chest is the major component of the aforementioned time delay, any changes in the refining action of the tickler refiner will be sensed sooner and corrective action will occur sooner than corresponding changes of refining action by the parallel refiners. Thus, if there is a deviation of overall refining action from set point, one would ideally desire that the maximum corrective action initially occur at the tickler refiner. Then, as the effect of subsequent control action at the parallel refiners is sensed, the loading on the tickler refiners could be reduced, such that, when a zero error, steady state condition was again achieved, the tickler refiner would be operating at approximately 50 percent of its capacity, thus insuring the later availability of the maximum hi-speed corrective action, in either direction, by the tickler refiner. Obviously with the prior art control system which controls all refiners to a fixed set point, it would be impossible to obtain the time sequencing of individual set points which this approach requires. However, through the use of the method and apparatus of our invention, this desirable control scheme may be realized.

Further, with the control method using individual control loops all controlling to a common set point, refiners with lower capacities could become overloaded, with a given refining set point, if flow increased since the load on the motor is determined by both the flow rate and the refining action desired. Thus, if the flow rate through the refiner increases to the point where the motor is overloaded, either the motor will be damaged or the refining action will decrease, deviating thereby from the set point. The approach of using individual control loops cannot adequately handle this contingency without the addition of other hardware such as flow limiting.

As a final example, it is probable, if not certain, that among a group of refiners there will be a difference in efficiency. Such differences may result from design differences, age, etc. Thus, one would ideally desire to have the more efficient refiners carry a greater share of the overall refining load-a result not achievable through use of individual control loops which load the refiners equally.

In summary, it has been pointed out that all prior art refiner control systems have had operational disadvantages. Through the use of our control system, all of these disadvantages may be eliminated.

SUMMARY OF INVENTION Our invention controls a group of refiners or a number of refiners within a group, as a system, rather than controlling each refiner on an individual basis. Our control system acts to adjust the refiners under its control such that the output of all the refiners in the group, when combined, will possess a desired quality such as freeness. Through the use of our control system, the refining requirement for the group can be allocated among the refiners under the control of the system on the basis of refiner capacity or the number of refiners in the group or the speed with which refiners respond or any combination of these or other criteria. Refiners which are in the group but not subject to adjustment by our control system may be locally controlled or base loaded.

A more complete understanding of our invention may be obtained from a consideration of the following comments and attached drawings.

DESCRIPTION OF DRAWINGS FIG. I is a flow diagram of part of the papermaking process.

FIG. 2 is a schematic drawing showing the relation of our invention to paper pulp refiners.

FIG. 3 is a generalized signal flow diagram of one embodiment of our invention.

FIG. 4 is a generalized signal flow diagram of one alternate embodiment of our invention.

FIG. 5 is a block diagram of one manner in which a paper pulp refiner could be instrumented for compatibility with our invention.

FIG. 6 is a block diagram functionally indicating one apparatus arrangement of our control system and the process steps to which various signals are subjected.

FIG. 7 is a block diagram of one specific embodiment of our invention and arranged in a manner which would facilitate analogue implementation.

FIG, 8 is an alternate representation of the control system and process presented in FIG. 7. The particular presentation of our invention in FIG. 8 would facilitate digital implementation.

DETAILED DESCRIPTION OF THE INVENTION FIG. I is a flow sheet representation of a refining process wherein a group of refiners, R R mR, are supplying refined stock to a machine chest 12 wherein the stock is mixed. From the machine chest 12 the stock passes through a tickler refiner 13 and then to the paper machine 14.

Each of the refiners, R R ...R,, receives stock from a conduit 10a, 10b...l0n respectively, and performs a certain amount of refining action on the stock which is related to the power input to the refiners, P P ...P,,. The refining action produced through the expenditure of the power input is manifested by a rise in temperature of the stock, which is generally represented in FIG. 1 and hereinafter referred to a AT (delta T) or AT AT ...AT, in FIG. 1. Those skilled in the art are aware that numerous commercially available transducers may be employed to measure either the power supplied to each refiner, P P ...P,,, or the temperature rise across each refiner, AT,, AT ...AT,,.

When applied to a refining process such as that shown in FIG. 1 our control system will control all of the refiners in the group R R ...R, or, alternatively, it will permit one or more of the refiners to be controlled locally. In either case, it will adjust each of the refiners under its control such that the combined output of all the refiners, including those not under its control, will have undergone the desired overall refining action.

FIG. 2 is an overall diagrammatic representation of a group of refiners and their relation to our control system. All lines in this FIG. are signal flow paths. All the refiners are feeding to one paper machine as was indicated in FIG. 1, and comprise a group, g. To illustrate the flexibility of our control system, we will assume that not all of the refiners in the group are under the control of our system. Those refiners which are in the group 3 and under the control of our system comprise a subgroup s of the group 3. Thus, refiners in the subgroup s comprise R R ...R, and the refiners in the group g comprise R R ...R,...R,,. A group of refiners operating as defined in this paragraph as well as the terminology in this paragraph will be used in all the discussions which follow.

From each of the refiners in the group g, viz R through R,,, signals representative of refiner operation, S through S, respectively, are sent to the refiner control system. Operating on these signals as well as other signals representing safety criteria, operating constants, etc. the control system will provide actuating signals AS through AS, to the refiners under system control, R through R,, such that the combined output of all the refiners in the group, R through R will have the desired quality, e.g. freeness.

FIG. 3 is a block diagram representation of our control system, each block functionally representing an apparatus with the function of the apparatus stated within the block. All lines indicate signals with arrows in the direction of signal flow except lines 15, 16, 17 and 18 which represent stock flow F F ...F ...F,,. From the following description of the generalized representation of our invention depicted in FIG. 3, it will be apparent to the skilled art worker that numerous different types of available hardware may be used to perform the functions indicated in each block of the block diagrams employed in FIG. 3 and later drawings. Thus, hereinafter, the discussion of each piece of hardware represented by any one block of a block diagram will be functional description. 1

In FIG. 3, refiners R through R, are operating subject to the criteria previously defined with reference to FIG. 2. Signals received from the refining operation are processed by an apparatus referred to as the Group Coefficient. of refining Computer. The single output signal, which we refer to as the Group Coefficient of Refining is directly related to the actual freeness of the combined stock from all the refiners and is representative of the overall or mean refining action of all the refiners in the group.

This signal is then compared to a set point signal which may be proportional to the desired freeness of the combined stock and is referred to in FIG. 3 as the desired Group Coefficient of Refining. The errors'ignal which results from this comparison, viz the group error signal, is directly related tothe deviation of actual freeness from desired freeness. This error signal is the input to the group controller which, operating from the group error signal, calculates the total control action required to restore the actual freeness to the desired freeness. Since the Group Control Signal is in the nature of an overall control signal, it must be distributed or allocated among the various refiners under system control, i.e. R R ...R,. This function is performed by the allocator which operates by applying an allocation factor to the Group Control Signal and producing thereby a desired coefficient of refining for each refiner under system control. In performing its allocation function, the al locator must be supplied with an allocation factor or factors. These factors are generated by the allocation factor computer in response to process conditions and operating constants. These factors may be automatically and continuously calculated or precalculated and set into the allocator.

Finally, the outputs of the allocator, i.e. the individual desired coefficients of refining, are compared with the individual, actual coefficients of refining. Any individual error signals are operated upon by a controller or controllers whose output is the actuating signals, AS AS AS ...AS to the individual refiners under system control R through R,.

Those skilled in the art will immediately perceive that our system, as generally described in FIG. 3, is amenable to both analogue and digital execution. Thus, it is to be noted that the bounds of our invention are not circumscribed by the equipment arrangements or processing steps indicated in FIG. 4 orlater drawings since the actual application of our invention may alter either equipment arrangements and/or processing steps. For example, with reference to FIG. 3, individual controllers are shown for each refiner under system control. However, at least two equivalents of our system would be (I) a single multiplexed analogue controller, or (2) a direct digital control system where each of the controllers shown in FIG. 3 was replaced by a control algorithm. These and other equivalents are within the ambit of the remaining discussion and drawings.

A somewhat more generalized embodiment of our invention is presented in FIG. 4 wherein the allocation and individual refiner control functions and apparatus are replaced by an element referred to in FIG. 4 as Group Error Signal Processor. Utilizing this approach, the Group Error Signal Processor,

which has available to it information as to the present operating levels of all the refiners, viz S,S,, and F -F,,, calculates new or desired values of S, viz S S 5,", which will reduce the Group Error Signal to zero. A typical manner in which this calculation could be effected would be to select a set of individual values of S and then iteratively utilize the equation employed to determine the Group Coefficient of Refining to obtain the desired individual values of S which would reduce the Group Error Signal to zero. In selecting a set of individual values of S, numerous criteria may be established; e.g. all values of S are equal; the values or S are related to refiner capacity, etc. After the Group Error Signal Processor calculates each of the desired refining action signals, 8,", S "...S, d, the difference between each desired refining action signal and the actual refining action signal is obtained, i.e. S -8,. This difference signal represents the amount and direction of desired change in the measured variable, 5,. Knowing the expression which relates a change in manipulated variable toa change in measured variable, individual actuating signals, A5,, AS ...AS,,, may be calculated since the actuating signals represent change in manipulated variable, e.g. disc separation in disc refining. The equation which represents this calculation is AS, m T (P i where, if each refiner responds linearly, T(p),- is the transfer function of the ith refiner.

Although the apparatus and process depicted by FIG. 4 is within the scope of invention, our preferred embodiments utilize the allocator and individual control approach shown in FIG. 3 since this. approach does not require one to know or determine the relationships between the various parameters of refiner operation, e.g. AT and disc separation. Thus, all our following examples utilizeto varying extents, the allocator and individual control approach. However, it should be unders'tood that all comments made in reference to subsequent examples or embodiments are applicable to the embodiment of FIG. 4 to the extent that they refer to equivalents of elements l9 and 20 ofFIG. 4.

FIG. 5 is a schematic flow chart of a typical refiner indicating the peripheral equipment associated with the refiner, e.g. the refine drive motor, equipment which may be used for adjusting the spacing between the moving surfaces, i.e. an hydraulic ram and associated pressure regulating equipment. Also indicated is one approach to instrumenting the refiner such that appropriate signals are made available to the control system.

As previously discussed, certain of the signals indicated in FIGS. 3 and 4 as going to the control system, are representative of the overall amount of refining done by the refiner on the pulp, e. g. drive motor current or AT. We refer to either of these or other similar signals as an index of refining action or as we have termed it, the coefficient of refining. When practicing our invention, one such coefficient of refining is selected, e.g. AT, and is sent to the control system where it is operated upon in conjunction with other such signals from the other refiners in the group g as will be hereinafter discussed.

FIG. 6 is a more quantitive description of our control system wherein a generalized transfer function is shown for each block within the system. Each transfer function represents both the function which its respective block is to perform as well as the process step which signal inputs to that block undergo. As shown, the particular coefiicient of refining on which the control system operates is. the temperature rise across the refiner or AT although other coefficients are available e.g. power consumption. The refiners from which the control system input signals were derived are a group of refiners as defined above in reference to FIG. 2. Further, an arbitrary refiner within the group will be referred to as the ith refiner with any signals associated therewith being denoted by the subscript i, e.g. AT, for the AT of the ithrefiner and F, as the stock flow through the ith refiner. The last refiner in the group is g and the last refiner is subclass s is s.

Using individual ATs as the individual coefficient of refining, we define the Group Coefficient of Refining as group AT or AT which may be generally stated to be a function of both the individual AT and the flow associated with each refiner, i.e. AT =f (AT AT ...AT,...AT,; F F,...F...F,,). After apparatus element 30 in FIG. 6 computes AT element 31 compares AT to the desired or set point AT viz AT Any difference between AT and AT gives rise to a group AT error signal, AT which is the input signal to the group controller 32. The group controller 32 operates upon AT to produce a group control signal, AT which is purely a function of AT i.e. AT =f(AT The group control signal AT must be allocated among the refiners in the group which are under system control. The result of this allocation, which is performed by apparatus element 33, is to generate an individual control signal, AT,- CS for each refiner under system control. In this generalized representation of our control system, the group AT control signal, AT is related to the individual AT control signal by an allocation factor A,-. The value of A, is supplied to apparatus element 33 by element 37 which calculates A,- on the basis of a program or strategy supplied to it. Although, as previously pointed out, various criteria may be imposed in the derivation of A we have in FIG. 6 derived A, as a function of stock flow rates through the refiners and weighting factor, i.e. A,=K f,(F,, F ...F...F,,) in the general case.

Apparatus elements 34, 35 and 36 represent the apparatus and process by which the allocated control signals, ATf are employed to adjust or control the individual refiners under system control, i.e. R through R,. Thus, the control system ultimately supplies individual activating signals, AT{ ...AT, which adjust the system controlled refiners such that their output when combined with the output of all refiners in the group but not under system control, will have the desired quality, e.g. freeness as manifested by AT being equal to AT FIGS. 7 and 8 present two specific embodiments of our invention.

The apparatus of FIG. 7 illustrates particular transfer functions which may be employed for each apparatus element or block. The control system shown in FIG. 7 is displayed in a manner which would make it particularly amenable to analogue implementation, i.e. each of the blocks with its appropriate transfer function are physically realizable through the use of standard and commercially available analogue equipment such as operational amplifiers and analogue controllers. In the following comments describing the operation of our invention as shown in the specific embodiment of FIG. 7, it is to be understood that we are controlling a refining system as heretofore defined in FIG. 2.

In this embodiment, the individual coefficient of refining utilized is AT. The control system receives from each refiner in the group g a signal representing individual AT, viz AT AT ..AT,, and a signal representing the flow through each refiner, viz F F mF. The first element or block in the control system 40 calculates the group coefficient of refining, viz group delta T or AT In this embodiment, we calculate AT as the flow weighted average of all the AT in the group g or:

The output of 40, viz AT is compared with the group delta T set point, AT by 41, the result of this comparison being the output of 41 or the group AT error signal, AT AT is the input to 42 which is shown as the equivalent of a three-function analogue controller, i.e. a device having a transfer function of the general form a+bp+c/p where a, b, and c are constants, viz positive real numbers but including zero and p is the Laplace operator, p indicating the time derivative and pindicating the time integral.

The output of 42 is the group AT control signal, AT the magnitude and polarity of which represents the control action required of the refiners under system control, i.e. the refiners in the subgroup s, to insure that the output of all refiners in the group g is such that AT =AT Those elements generally represented by 43 together with 44 allocate the group control signal AT to generate individual control signals AT AT AT Each of the individual control signals, AT, is related to the group control signal, AT by an allocation factor A,-. In this embodiment all allocation factors are equal, i.e. A 0.14,. A,- is calculated by 44. 44 receives signals, F,, F F,,, representing the flow through each refiner, each of these signals being the same as the flow signals that were supplied to 40. 44 is also programmed or supplied with information as to the number of refiners which are in the group, g, and which refiners in the group g are in the subgroup s, i.e. which refiners are under system control. On this basis, 44 computes A, according to the equation:

Thus, in this embodiment, it will be appreciated that A,- is equal to the ratio of the total flow through all the refiners in the group g to the flow through all the refiners in the subclass s. As to the numerical value of A in the general case when g s, it follows that A 1. In the limiting case of g=s, then A=1. As to the permissible value ofs, we define that s 0, i.e. at least one refiner is always under system control.

The individual control signals, AT, essentially bias initial set point signals, AT through the use of summing junctions, generally indicated by 45. The output of the individual summing junctions which comprise 45 are the new individual delta T set points for each refiner, viz A71 AT "...AT The individual AT set points, ATF are compared with each individual, actual AT, AT by comparators generally represented by 46 which generate individual error signals, AT ATf...AT, Each of the individual AT error signals, AT,", is the input to individual controllers generally represented by 47 and having transfer functions of the type defined hereinbefore for 42. The output of the individual controllers are the actuating signals, AT which are applied to individual refiner actuators such as the adjusting motor which adjusts the pressure regulator controlling the hydraulic ram.

A digital execution of our invention, one representation of which is presented in FIG. 8, represents our preferred embodiment. Those skilled in the art will appreciate our reference for the use of a digital computer to implement our invention since the flexibility of our system can be most completely achieved by this approach.

FIG. 8 presents our invention in the nature of program flow sheet. Functionally, the process indicated by the flow sheet is equivalent to the apparatus implementation shown in FIG. 7, i.e. AT is used as the coefficient of refining and AT is the group coefficient of refining. Additionally, however, we have incorporated into our preferred embodiment a method of automatically transferring refiners out of the subgroup s. Such an automatic transfer is desirable since one would not wish to send control signals to a refiner if one of the operating variables such as flow, F, hydraulic pressure, P, motor current, I, or AT had exceeded its respective safety or operational limit.

Prior to commencing operation, the operator would enter into memory information as to which refiners in the group g are under system control, i.e. which refiners in the group g are in the subgroup s. Also entered into memory would be the group AT setpoint, AT initial individual AT setpoints At and all safety limits. As indicated in FIG. 8, the variables of interest for our refiner are scanned 48 and stored 49 for future readings. The stored data is then read 50 and checked 52 against the safety limits which are read 51 from memory. If any variable exceeds its limit we check 53 to see if that refiner is in the subgroup s. If the out of limit refiner is in the subgroup s, the machines memory is altered 54, i.e. that refiner is taken out of s. Irrespective of whether the out of limit refiner is in s, we alarm print, 55 to inform the operator of this out of limit status. Since the alarm print on every cycle would place an excess burden on the alarm typewriter we inhibit the alarm printout through time check 55 whereby the alarm printout is limited to, e.g. one print every 3 minutes.

After all the variables for one refiner (the i refiner) have been checked against their limits, we check 56 to see if all the refiners in the group have undergone a safety limit check. All refiners will have been checked when i=g. If all refiners have not been checked we recycle 57 for the next refiner, viz i+1, and continue until i=g.

When all refiners have undergone a safety limit check, i.e. when i=g, we proceed to calculate the allocation factor A. This is accomplished by scanning all flow inputs 58, obtaining from memory the present status as to the group g and the subclass s, 59, and then calculating A, 60, in accordance with the following:

After A is calculated it is stored 61 in memory for future use.

As the next step in the process we read 62 all delta ATs and flows associated with each refiner in g and calculate 63 the group AT, AT as follows:

Following the calculation of AT we read 64 the group AT setpoint, AT from memory and compare it to AT 65. The result of this comparison, which is AT or the group AT error signal, is used to calculate 66 the group control signal, AT as follows:

AT =f(a, b,c, AT (t), AT (!l),...) where AT (t) is the present Group ATError Signal and AT; (1-1) is the Group AT Error Signal during the previous time increment. The right-hand side of the above equation is a generalized expression representing a three-mode control velocity algorithm.

Following the calculation of AT we proceed to develop an actuating signal for each refiner in the group. One of two methods is employed to develop the individual actuating signal, depending on whether the individual refiner is in the subgroup s, i.e. is i in s. In general, if any refiner, i, is not in s, it is controlled to a fixed set point irrespective of AT However, if i is in s then control is responsive to AT More specifically, after we calculate AT t all the refiners in g are considered sequentially. For each refiner we check 67 whether i is in s. If i is not in s, we proceed to read 68 from memory the AT set point for the refiner under consideration, AT, We then read 69 the actual AT for that refiner AT, and compare it 70 to ATf", the result being error signal for the refiner, ATf. Using AT, we calculate 71 the individual actuat ing signal, AT,- using an equation of the form employed to calculate AT If i is in s, we proceed to calculate 73 a new AT set point by reading 72 the value of A the allocation factor, from memory and calculating 73 an individual control system, AT, follows:

AT, A AT After calculating AT, we read 74 the value of the last set point for that refiner from memory and then calculate 75 a new AT set point for that refiner, this new set point being defined as:

new AT, AT, last AT, The new AT,- as calculated above is then stored 76 in memory, in place of the last AT,- for future use. With the new AT, available in memory, we read 77 the actual value of ATfor the refine and, upon reading 78 the new AT, from memory, we compare 79 AT, with the actual ATfor that refiner, AT,. The error signal created by this comparison AT,-" is used to calculate 71 the individual actuating signal, AT as heretofore described.

After we actuate the refiner 80 in response to AT, we check 81 to see if the refiner just actuated was the last refiner in the group, i.e. is i=g. If it is not the last refiner in the group,

the individual control process steps 67 to 71 or steps 72-79, and 71 is recycled 82 for the next refiner, viz refiner i+1. The recycle of the individual control process is continued until all refiners in the group have been actuated at which time i=g. When Pg, we then recycle 83 to the start of the entire process at step 48, the recycle being controlled and actuated by clock 84.

Although we have recited herein numerous embodiments of our invention, such recitations have been by way of example and not by way of limitation. Because of the intrinsic flexibility of invention, as indicated by our various examples, numerous modifications of our invention will be evident to those skilled in the art, all of which fall within the scope of our appended claims.

We claim:

1. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel comprismg:

a. means for obtaining from each of said refiners at least one signal representing the temperature difference across each refiner;

b. means for obtaining from each of said refiners at least one signal representing the flow of stock through each of said refiners;

means for combining said signals representing the temperature difference across each of said refiners and said signals representing the flow of stock through each of said refiners into a first signal representing the flow weighted average temperaturedifference across all of said refiners;

d. means for providing a second signal representing desired flow weighted average temperature difference across all of said refiners;

e. means for comparing said first and second signals to obtain a difference signal; and

f. control meansfor controlling at least one of said refiners in response to said difference signal.

2. A control system for controlling the quality of the outpu stock of a group of pulp refiners operating in parallel which comprises:

a. means for obtaining a plurality of signals, each representative of the refining action of each refiner in said group;

b. means for combining said plurality of signals to obtain a first signal representative of the mean refining action of all of said refiners;

c. means for generating a second signal representative of desired mean refining action;

(1. means for comparing said first signal with said second signal to develop an error signal;

e. means for calculating a group control signal in response to said error signal;

f. means for generating at least one group control signal allocation factor;

g. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factor;

b. means for altering at least one individual refining action set point in response to said individual control signal; and

i. means for controlling the refining action of at least one individual refiner in said group in response to the deviation of actual refining action from said individual refining action set point. i

3. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel which comprises:

a. means for obtaining a plurality of signals, each representative of the temperature difference across each refiner in said group;

b. means for combining said plurality to obtain a first signal representative of the mean temperature difference across all of said refiners;

c. means for generating a second signal representative of desired mean temperature difference;

d. means for comparing said first signal with said second signal to develop an error signal;

e. means for calculating a group control signal in response to said error signal;

f. means for generating at least one group control signal allocation factor; I

g. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factor;

h. means for altering at least one individual temperature difference set point in response to said individual control signal; and

i. means for controlling the temperature difference across at least one individual refiner in response to the deviation of the individual temperature difference from said individual temperature difference set point.

4. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel which comprises:

a. means for obtaining a first plurality of signals, each representative of the temperature difference across each refiner in said group;

b. means for obtaining a second plurality of signals, each representative of the flow through each refiner in said group;

c. means for combining said first plurality and said second plurality of signals to obtain a third signal representative of the flow weighted average temperature difference across all of the refiners in said group;

d. means for generating a set point signal;

e. means for comparing said third signal with said set point signal to develop an error signal;

f. means for calculating a group control signal in response to said error signal;

g. means for generating group control signal allocation factors;

h. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factors;

i. means for altering at least one individual temperature difference set point in response to said individual control signals; and

j. means for controlling the temperature difference across at least one individual refiner in response to the deviation of the individual temperature differences from said individual temperature difference set point.

5. The control system of claim 4 wherein all of said allocation factors are equal.

6. The control system of claim 5 wherein said allocation factor equals the ratio of the total flow of stock through all the refiners in the group to the total flow of stock through all the refiners which are in the group and are controlled in response to the group control signal.

7. I The method of controlling a group of pulp refiners operating in parallel which comprises:

a. obtaining from each refiner in said group of refiners at least one signal which is representative of the refining station of each of said refiners;

b. combining each of said signals from each of said refiners into a first signal representative of overall refining action;

c. obtaining a second signal representative of desired overall refining action;

d. comparing said first and second signals to obtain an error signal;

. producing a group control signal in response to said error signal; obtaining at least one group control signal allocation factor;

g. generating at least one individual refining action set point signal from said group control signal in response to said allocation factor;

h. comparing said individual refining action set point signal to a signal representing the actual refining action of at least one of said refiners to obtain a second error signal;

and

. controlling at least one of said refiners in response to said second error signal whereby the overall refining of all of said refiners in the group is equal to the desired overall refining action.

8. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel comprismg:

a. means for obtaining from each of said refiners at least one signal representative of the refining action of each refiner in said group;

b. means for obtaining from each of said refiners at least one signal representative of the flow of stock through each refiner in said group;

0. means for combining said signals representative of the refining action of each of said refiners and said signals representative of the flow of stock through each of said refiners into a first signal representative of the mean refining action of all of said refiners;

d. means for generating a set point signal representative of the desired mean refining action;

e. means for comparing said first signal with said set point signal to develop an error signal; and

f. means for controlling at least one of said refiners in response to said error signal.

9. The apparatus of claim 8 wherein said signal representative of the refining action of each refiner in said group is representative of the power consumption of the refiner drive motor.

10. The apparatus of claim 8 wherein said signal representative of the refining action of each refiner in said group is representative of the temperature difference across each of the refiners in the group.

11. The apparatus of claim 10 wherein said means for controlling includes means for varying the loading on one of the refining surfaces.

12. The apparatus of claim 10 wherein said means for controlling includes means for varying the spacing between the refining surfaces.

12%?" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent '%.')68,939 Dated M r h 9, 197

Inventot(s)D.B. Brewster, P.H. Emery, Jr. R.C. Hatcher 80 0.5

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, Line 6 'characterized" should be "industrial" Column 1, Line 50, "comprises" should be "comprised" Column 2, Line 63,"rela.tes" should be "related" Column 5, Line lO,"F .F. .F should be "F .F .F Column 6, Line l3,"S s ...s d" should be s s ...s

Column 7, Line 5',"F F2, .F. .F should be "F 3 .F

I Column 7, Line 55,"F F2. .F" should be "F F2. .Fg Column 8, Line 1&6, "reference" should be "preference" H s I! Column 8, Line 65," A t should be T p Column 11, Line 55-60, "station" should be "action Signed and sealed this 6th day of July 1971.

(SEAL) Atteat: r

JR CHER JR. WILLIAM E. SOHUYLER, figifs ifig icer Gonmiasianer of Patents 

2. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel which comprises: a. means for obtaining a plurality of signals, each representative of the refining action of each refiner in said group; b. means for combining said plurality of signals to obtain a first signal represeNtative of the mean refining action of all of said refiners; c. means for generating a second signal representative of desired mean refining action; d. means for comparing said first signal with said second signal to develop an error signal; e. means for calculating a group control signal in response to said error signal; f. means for generating at least one group control signal allocation factor; g. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factor; h. means for altering at least one individual refining action set point in response to said individual control signal; and i. means for controlling the refining action of at least one individual refiner in said group in response to the deviation of actual refining action from said individual refining action set point.
 3. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel which comprises: a. means for obtaining a plurality of signals, each representative of the temperature difference across each refiner in said group; b. means for combining said plurality to obtain a first signal representative of the mean temperature difference across all of said refiners; c. means for generating a second signal representative of desired mean temperature difference; d. means for comparing said first signal with said second signal to develop an error signal; e. means for calculating a group control signal in response to said error signal; f. means for generating at least one group control signal allocation factor; g. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factor; h. means for altering at least one individual temperature difference set point in response to said individual control signal; and i. means for controlling the temperature difference across at least one individual refiner in response to the deviation of the individual temperature difference from said individual temperature difference set point.
 4. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel which comprises: a. means for obtaining a first plurality of signals, each representative of the temperature difference across each refiner in said group; b. means for obtaining a second plurality of signals, each representative of the flow through each refiner in said group; c. means for combining said first plurality and said second plurality of signals to obtain a third signal representative of the flow weighted average temperature difference across all of the refiners in said group; d. means for generating a set point signal; e. means for comparing said third signal with said set point signal to develop an error signal; f. means for calculating a group control signal in response to said error signal; g. means for generating group control signal allocation factors; h. means for generating at least one individual control signal by operating on said group control signal in response to said allocation factors; i. means for altering at least one individual temperature difference set point in response to said individual control signals; and j. means for controlling the temperature difference across at least one individual refiner in response to the deviation of the individual temperature differences from said individual temperature difference set point.
 5. The control system of claim 4 wherein all of said allocation factors are equal.
 6. The control system of claim 5 wherein said allocation factor equals the ratio of the total flow of stock through all the refiners in the group to the total flow of stock through all the refiners which are in the group and are controlled in response to the group control signal.
 7. The method of controlling a group of pulp refiners operating in parallel which comprises: a. obtaining from each refiner in said group of refiners at least one signal which is representative of the refining station of each of said refiners; b. combining each of said signals from each of said refiners into a first signal representative of overall refining action; c. obtaining a second signal representative of desired overall refining action; d. comparing said first and second signals to obtain an error signal; e. producing a group control signal in response to said error signal; f. obtaining at least one group control signal allocation factor; g. generating at least one individual refining action set point signal from said group control signal in response to said allocation factor; h. comparing said individual refining action set point signal to a signal representing the actual refining action of at least one of said refiners to obtain a second error signal; and i. controlling at least one of said refiners in response to said second error signal whereby the overall refining of all of said refiners in the group is equal to the desired overall refining action.
 8. A control system for controlling the quality of the output stock of a group of pulp refiners operating in parallel comprising: a. means for obtaining from each of said refiners at least one signal representative of the refining action of each refiner in said group; b. means for obtaining from each of said refiners at least one signal representative of the flow of stock through each refiner in said group; c. means for combining said signals representative of the refining action of each of said refiners and said signals representative of the flow of stock through each of said refiners into a first signal representative of the mean refining action of all of said refiners; d. means for generating a set point signal representative of the desired mean refining action; e. means for comparing said first signal with said set point signal to develop an error signal; and f. means for controlling at least one of said refiners in response to said error signal.
 9. The apparatus of claim 8 wherein said signal representative of the refining action of each refiner in said group is representative of the power consumption of the refiner drive motor.
 10. The apparatus of claim 8 wherein said signal representative of the refining action of each refiner in said group is representative of the temperature difference across each of the refiners in the group.
 11. The apparatus of claim 10 wherein said means for controlling includes means for varying the loading on one of the refining surfaces.
 12. The apparatus of claim 10 wherein said means for controlling includes means for varying the spacing between the refining surfaces. 